1. Field of the Invention
The present invention relates to a zoom lens in which the lens unit located closest to the object side has a negative refracting power. The present invention also relates to an image pickup apparatus, such as a digital camera, a video camera, or a camera-equipped cellular phone, equipped with a zoom lens.
2. Description of the Related Art
It has been conventionally known that a type of zoom lens in which the lens unit located closest to the object side has a negative refracting power is more advantageous in achieving a large angle of view and size reduction than a type of zoom lens in which the lens unit located closest to the object side has a positive refracting power. In addition, zoom lenses in which the lens unit located closest to the object side has a negative refracting power is also advantageous in reducing the thickness (i.e. the length along the optical axis) of the zoom lens in the state in which the lens barrel is collapsed, and therefore such zoom lenses are used, for example, as taking zoom lenses of compact digital cameras.
Among this type of zoom lenses in which the frontmost lens unit has a negative refracting power, there have been known zoom lenses in which the number of lenses in the first lens unit having a negative refracting power is as small as two, and the number of lenses in the second lens unit having a positive refracting power is four, whereby good optical performance is achieved (see patent documents 1 to 9 listed below).
The zoom lenses disclosed in patent documents 1 to 9 have the first lens unit (i.e. the frontmost lens unit) composed of two lens elements including a negative lens and a positive lens. This lens configuration is advantageous in reducing chromatic aberration in the first lens unit and in reducing the size of the zoom lens with respect to the diametrical direction. Furthermore, the negative lens in the first lens unit is designed to have a biconcave shape in the paraxial region, whereby the negative refracting power of the negative lens on the optical axis is shared by the object side surface and the image side surface. This is advantageous in reducing spherical aberration. Still further, the second lens unit is composed of four lens elements. This provides a sufficient degree of freedom in the design of the second lens unit and is advantageous in achieving good correction of aberrations in a small size zoom lens.
In recent years, image pickup apparatuses that use a CCD or CMOS sensor have replaced film cameras to become the mainstream. The size of image pickup apparatuses such as digital cameras and video cameras can easily be made small because the image pickup area thereof is relatively small.
In the field of digital cameras and video cameras that use a small image pickup element, there is a demand for image pickup apparatuses that is small in the thickness direction and has a high zoom ratio and a wide angle of view. There are known zoom lenses that are intended to be used in small-size image pickup apparatuses, as disclosed in, for example, patent documents 10 to 13 listed below.    Patent Document 1: Japanese Patent Application Laid-Open. No. 2003-114386    Patent document 2: Japanese Patent Application Laid-Open. No. 2005-258059    Patent Document 3: Japanese Patent Application Laid-Open. No. 2006-003539    Patent Document 4: Japanese Patent Application Laid-Open. No. 2006-195064    Patent Document 5: Japanese Patent Application Laid-Open. No. 2006-201599    Patent Document 6: Japanese Patent Application Laid-Open. No. 2006-227197    Patent Document 7: Japanese Patent Application Laid-Open. No. 2007-114727    Patent Document 8: Japanese Patent Application Laid-Open. No. 2007-140359    Patent Document 9: Japanese Patent Application Laid-Open. No. 2007-272216    Patent Document 10: Japanese Patent Application Laid-Open. No. 2003-241091    Patent Document 11: Japanese Patent Application Laid-Open. No. 11-52246    Patent Document 12: Japanese Patent Application Laid-Open. No. 2006-194957    Patent Document 13: Japanese Patent Application Laid-Open. No. 2008-46529